1. Field of the Invention
The invention relates to a multilayer material for sliding elements and to the use and production of such a multilayer material.
2. Description of Related Art
Bearing materials with plastics-based overlays are known as single-layer, two-layer or three-layer composite materials: solid plastics bearings, bearings with an outer metallic backing and directly applied or adhered plastics, other such with inner wire meshes, as well as three-layer bearings of backing metal, a sintered porous metal layer and a covering layer formed in the pores. The multilayer materials differ from solid plastics materials, for example by a negligible tendency towards cold flow under load, by substantially better heat conductivity and, in connection therewith, by notably higher possible pv values.
Among three-layer materials, it is possible to distinguish further between those with overlays based on fluorothermoplastics, such as PTFE, PFA, FEP etc., or other plastics such as PEEK, for example. The latter two groups differ in their manner of operation: while, in the case of PTFE-based materials, the bronze intermediate layer is the load-bearing component of the overlay and acts like a filler, the other plastics materials use it only as an anchoring means. On the sliding surface itself the thermoset or thermoplastics material assumes the supporting role of the bronze. If there is sufficient affinity to the metal backing, mechanical anchoring in the bronze framework is also superfluous and the production of true two-layer materials is possible.
The production of such two-layer materials may be advantageous owing to a reduction in costs connected therewith, but it also opens up the possibility of using different backing materials from usual, since their use here is not dependent on the adhesion of the porous bronze layer conventionally sintered thereto.
A PVDF/PTFE-based material is known from DE-OS 19 25 000 which is described only as a solid plastics material and contains as an additional additive from 5 to 35% of low-molecular weight fluoropolymers. These are described as being indispensable if adequate stability of the moulding is to be achieved. The material may also contain 10-25 parts of non-metallic fillers, in order to improve wear properties.
DE-OS 18 06 551 proposes a method of coating metal sheets which are for use in building and must therefore comprise suitable corrosion protection. Between the metal sheet and a PVF coating it is necessary to provide a primary coupling layer of metal oxides and PVF to aid bonding.
DE-OS 24 36 424 describes a method by which adhesion between fluoroplastics, which are used without additives, and a metal substrate is improved by the breakdown of internal stresses. This is described as follows: the fluoroplastics are held on the substrate firstly for a few minutes above melting point, then for a few minutes just below melting point. These materials are not used for sliding elements.
These prior publications all comprise the use of additional coupling agents or elaborate heat treatment. It should also be noted that a layer whose adhesion is substantially impaired by internal stress is unsuitable for the conventional process of producing plain bearings coated with plastics material, since considerable deformation may have to be undertaken after coating without this resulting in a loss of adhesive power.
In EP 0 340 839 B1 a powder mixture consisting of PVDF, PTFE and lead or another metal with a high affinity for fluorine is applied to a metal strip and then exposed for from 5 to 50 mins to a temperature above the decomposition temperature of PVDF, whereby particularly good tribological properties are said to be achieved.
DE-OS 29 28 081 relates to the production of two-layer materials of steel, a PVDF-based plastics overlay being applied. PTFE, MoS2 or graphite are used as additives. Moreover, additional substances with a density xe2x89xa78 g/cm3 have also to be provided in this bearing material in an amount of from 5 to 35%. These are essentially metal components such as lead, lead oxide and the like, which are intended to increase surface bonding to the substrate. It is further proposed, with the aid of the solvent DMF, to produce a dispersion-type mixture from the overlay components and to apply it to the metal backing.
All these materials have the disadvantage that poisonous metals or metal compounds are provided and their production is effected with the aid of solvents or even by releasing poisonous decomposition products.
A bearing material similar to that in DE-OS 2928081 is described in EP 0 430 324 A1. It consists of 5-30% PTFE, 5-60% bronze and 1-20% graphite in a PVDF matrix. The disadvantage of this bearing material consists in the fact that, from the beginning, a bronze component is constantly involved in the friction process. In the case of matrix materials such as PVDF, this impairs dry running properties.
Therefore, the description of this invention mentions only lubricated applications, especially shock-absorbers.
The problem on which the invention is based is that of providing a multilayer material which is characterised by good adhesion to the substrate without the disadvantages of the known PVDF-based bearing materials, as well as a method for the production thereof.